The present invention relates to a process for making silica crucibles and other articles from finely divided silica particles and more particularly to the manufacture of sintered vitreous silica receptacles having exceptional resistance to spalling, blistering or deterioration during devitrification. The process makes it possible to mass produce precision transparent or translucent vitreous silica crucibles which remain transparent or avoid deterioration when devitrified in contact with molten silicon during the growing of a silicon crystal, thereby avoiding contamination and permitting formation of a silicon crystal of highest quality.
Heretofore, vitreous silica articles have been fabricated by glass working, slip casting, pressing, or the like. Glass working methods involve heating the glass to a temperature, such as 3500.degree. F. or higher to lower the viscosity to a point where the glass may be subjected to plastic deformation, as by rolling, pressing or drawing. Slip casting has also been used extensively for making a variety of vitreous silica articles, particularly refractory articles having a density below 90 percent of theoretical which are to be subjected to intermediate temperatures below 2000.degree. F. In recent years, slip casting has also been used for making high density vitreous silica articles, such as crucibles for growing of silicon crystals.
Hot pressing and other pressing operations are not used extensively in the commercial fabrication of common vitreous silica products, but pressing techniques can be used for such purposes and can be used to produce sintered full density silica products.
For many years, prior to the present invention glass working produced the best crucibles for use in the growing of silicon crystals. Sintered crucibles formed by slip casting have also been used because of lower cost of manufacture, but they have been considered inferior to the blown-glass crucibles.
Crucibles have also been made by the arc-fusion process in which quartz sand is melted at a high temperature above 3500.degree. F. while positioned against a graphite mold by centrifugal force. The arc-fusion process avoids substantial devitrification and makes it possible to produce an amphorous silica with good thermal shock resistance, but the unmelted sand provides the resulting crucible or other silica glass products with a rough surface which is undesirable. The crucibles made by the arc-fusion process are clearly inferior to the high quality crucibles which can be made by slip casting according to U.S. Pat. No. 3,837,825, for example.
As pointed out in the latter patent the manufacturing process must be properly controlled to make possible manufacture of high quality transparent silica crucibles or other articles by a sintering operation because of the nature of silica and the need to avoid formation of substantial amounts of cristobalite. The latter undergoes the alpha-beta inversion at a temperature between 340.degree. and 510.degree. F. and, when present in relatively small amounts, such as a few percent by weight, causes the silica to crack or deteriorate when it reaches the inversion temperature during cooling of the sintered product. Because the rate of formation of cristobalite increases rapidly when vitreous silica is heated above 2200.degree. F.*, it has been a common procedure to fire slip cast products at temperatures not substantially greater than 2200.degree. F. FNT *The rate of formation of cristobalite at 2400.degree. F. is perhaps at least ten times the rate of formation at 2200.degree. F.
One technique which reduces the cristobalite problem is to raise the temperature somewhat above the melting point of cristobalite at the end of the sintering operation, thereby destroying the cristobalite formed during sintering. This does, however, result in flow of the glass and requires a supporting mandrel to maintain the desired shape as disclosed, for example, in U.S. Pat. No. 3,763,294.
In the case of vitreous silica crucibles used to hold molten silicon during the growing of silicon crystal rods, the crucible was considered undesirable if it contained substantial amounts of cristobalite. At the present time, and for many years prior to this invention, cristobalite has been considered undesirable by those that manufacture silicon crystal rods, because of the blistering or deterioration which normally results during formation of cristobalite. The present invention contradicts the established concepts and involves a discovery which revolutionizes the industry by avoiding such deterioration.
In the typical process for the manufacture of silicon crystal rods, a crystal growing apparatus is employed which may, for example, be of the general type disclosed in U.S. Pat. No. 3,094,006. In the crystal growing process, the vitreous silica crucible contains the molten silicon from which the crystal rod is grown. The silicon in the crucible is initially heated for about one-half hour to a temperature of about 2585.degree. F. A seed crystal is introduced into the molten silicon and the silicon crystal is pulled for a period of about 8 to 16 hours at about 2585.degree. F.
Heretofore, a transparent amorphous quartz glass crucible, even if formed essentially free of cristobalite, would deteriorate during this operation by devitrification. The presence of even relatively small amounts of impurities or a small amount of cristobalite in the crucible at the beginning of the heating period was considered highly undesirable because it accelerated this deterioration. Therefore, it was considered essential, prior to this invention, to employ transparent silica crucibles of highest quality -- preferably pure quartz glass crucibles made by glass working, for example, formed from quartz glass tubing.
In recent years, slip cast crucibles have been produced according to said Pat. No. 3,837,825 with a very high quality, but these were generally considered inferior to blown-glass crucibles up to the time of the present invention. In accordance with said patent, vitreous silica crucibles of high quality are made from particles of fused silica which are molded or formed by slip casting or pressing. A slip-cast crucible, for example, which has been dried and fired at a high temperature such as 2100.degree. F. is heated rapidly to a high temperature, such as in the range of 3000.degree. F to 3100.degree. F. or higher, and maintained at a temperature in that range in a furnace for a short period of time, such as 1 to 5 minutes, to sinter the fused silica and to increase the density to 99 percent or greater while avoiding significant devitrification and maintaining the desired shape. Such heating may, for example, be carried out in a furnace containing an inert gas, such as helium, nitrogen or argon. Where the vitreous silica article is to be transparent, the sintering may be carried out in a vacuum or in an atmosphere of helium. of the type disclosed in said Pat. No. 3,837,825 made it possible to produce transparent crucibles of extremely high quality. The reported inferiority of the slip-cast crucibles as compared to glass-worked or blown-glass crucibles was perhaps due more often to chemically-bound water in the silica glass than to a high cristobalite content or impurities in the glass. The typical slip-cast and blown-glass crucibles being manufactured for crystal growing contained a substantial amount, such as 100 parts per million or more, of chemically bound water which is trapped in the glass and can not be removed during heating because of the impermeable nature of glass.
Those skilled in the art were not aware of the significance of such small amounts of chemically bound water and continued to produce slip-cast and blown-glass crucibles which had an excessive water or hydroxyl content and were subject to spalling and blistering during devitrification. The deterioration which occurred during the formation of cristobalite resulted in contamination of the molten silicon during crystal growing and sometimes catastrophic crucible breakage, but such deterioration was accepted for two decades as an inherent and inevitable consequence of devitrification.
The need to remove chemically bound water and the importance of low water content were not recognized prior to this invention and apparently were not recognized long after the invention was in commercial use because competitors continued to make inferior slip-cast crucibles with an excessive water or hydroxyl content and manufacturers of silicon crystal rods continued to consider cristobalite as the problem rather than water content. Many of such manufacturers still fail to understand how a blown-glass crucible can be inferior to a slip-cast crucible.